linux-kernel - Re: [sched next] overflowed cpu time for kernel threads in /proc/PID/stat

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Message-ID: <20130903131557.GA2276@swordfish.minsk.epam.com>
Date:	Tue, 3 Sep 2013 16:15:57 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Subject: Re: [sched next] overflowed cpu time for kernel threads in
 /proc/PID/stat

On (09/03/13 10:43), Stanislaw Gruszka wrote:
> > > > Thanks a lot Sergey for testing this further!
> > > > 
> > > > Interesting results, so rtime is always one or two units off stime after scaling.
> > > > Stanislaw made the scaling code with Linus and he has a better idea on the math guts
> > > > here.
> > > 
> > > I don't think this is scale issue, but rather at scale_stime() input
> > > stime is already bigger then rtime. Sergey, could you verify that
> > > by adding check before scale_stime() ?
> > > 
> > 
> > usually stime < rtime.
> > this is what scale_stime() gets as input:
> > 
> > [ 1291.409566] stime:3790580815 rtime:4344293130 total:3790580815
> 
> Ok, I see now, utime is 0 . This seems to be problem with dynamic ticks
> as you told that your application is kernel compilation, so we utilize
> lot of cpu time in user-space.
> 
> Anyway we should handle utime == 0 situation on scaling code. We work
> well when rtime & stime are not big (variables and results fit in
> 32 bit), otherwise we have that stime bigger than rtime problem. Let's
> try to handle the problem by below patch. Sergey, does it work for you ?

works fine on -next.

	-ss

> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index a7959e0..25cc35d 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -557,7 +557,7 @@ static void cputime_adjust(struct task_cputime *curr,
>  			   struct cputime *prev,
>  			   cputime_t *ut, cputime_t *st)
>  {
> -	cputime_t rtime, stime, utime, total;
> +	cputime_t rtime, stime, utime;
>  
>  	if (vtime_accounting_enabled()) {
>  		*ut = curr->utime;
> @@ -565,9 +565,6 @@ static void cputime_adjust(struct task_cputime *curr,
>  		return;
>  	}
>  
> -	stime = curr->stime;
> -	total = stime + curr->utime;
> -
>  	/*
>  	 * Tick based cputime accounting depend on random scheduling
>  	 * timeslices of a task to be interrupted or not by the timer.
> @@ -588,13 +585,19 @@ static void cputime_adjust(struct task_cputime *curr,
>  	if (prev->stime + prev->utime >= rtime)
>  		goto out;
>  
> -	if (total) {
> +	stime = curr->stime;
> +	utime = curr->utime;
> +
> +	if (utime == 0) {
> +		stime = rtime;
> +	} else if (stime == 0) {
> +		utime = rtime;
> +	} else {
> +		cputime_t total = stime + utime;
> +
>  		stime = scale_stime((__force u64)stime,
>  				    (__force u64)rtime, (__force u64)total);
>  		utime = rtime - stime;
> -	} else {
> -		stime = rtime;
> -		utime = 0;
>  	}
>  
>  	/*
> 
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